Pigment dispersions for pigmented phase change ink (sometimes referred to as hot melt ink or solid ink) require milling of pigments to a particle size preferably as small as the primary particle size of the pigment which can be 100 nanometers in diameter or less. Pigment size reduction can be costly and energy intensive. Known technologies that are suitable for dispersing pigments in wax media at temperatures higher than 100° C. include attrition, extrusion and homogenization. These processes require excessive milling times (up to 7 days milling time) and in some cases known processes are not even capable of delivering the required pigment particle size. Furthermore, in processes that utilize milling media (stainless steel, silica, zirconia, etc.) media wear can result in ink contamination.
Pigment dispersion is an important step in the manufacturing of pigmented phase change ink. Pigments are often supplied by the manufacturer as dry aggregates and agglomerates many times larger than the primary particle size (usually on the order of less than about 100 nanometers in diameter). These aggregates or agglomerates have to be reduced in size to less than about 100 nanometers in diameter with a narrow particle size distribution to form a stable dispersion in the solid ink medium. This is needed for the resulting solid ink formulation to demonstrate good jetting and print quality performance.
Processes for particle dispersion and pigment size reduction, such as milling and homogenization, are known. These known processes operate at room temperature, or at elevated temperatures, but still significantly below 100° C.
Wax dispersions used for manufacturing emulsion aggregation toners can be prepared by emulsifying a molten polyethylene or polymethylene wax in de-ionized water and surfactant using a piston homogenizer operating at 6,000 psig pressure and at a temperature of 120° C. to 130° C. The resulting emulsion typically contains about 30 weight percent solids and the following particle size specifications:
D50 Spec: 209±45 via W143 Nanotrac® (243±nanometers via Omnova Brookhaven)
D95 Spec: 650 nanometer maximum
U.S. patent application Ser. No. 13/940,731, which is hereby incorporated by reference herein in its entirety, describes in the Abstrate thereof a process for preparing a pigment dispersion including melting a phase change ink carrier and an optional dispersant at a temperature of from about 100° C. to about 170° C. in a jacketed vessel comprising a jacket surrounding an immersion mill or basket mill attachment and a dispersion blade attachment disposed within the jacketed vessel; mixing the melted phase change ink carrier and optional dispersant with a dispersion blade in the jacketed vessel; adding a pigment to the melted, mixed phase change ink carrier and optional dispersant in the jacketed vessel; wetting the pigment; after pigment wetting, reducing the pigment particle size using an immersion mill or basket mill attachment in the jacketed vessel; and discharging the pigment dispersion.
The appropriate components and process aspects of the each of the foregoing U. S. patents and Patent Publications may be selected for the present disclosure in embodiments thereof. Further, throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.